1. Field of the Invention
The present invention relates in general to a scan format of a video signal, and more particularly to an apparatus and a method for converting a scan format of a video signal, in which an input signal of a raster scan format is converted into that of a vertical scan format easy to calculate.
2. Description of the Prior Art
As well known, the Moving Pictures Experts Group (referred to hereinafter as MPEG) initially proposed MPEG1 (formal name: ISO/IEC 1172) in the 1.5 Mbps or more class based on storage media. Thereafter, MPEG2 in the 5 Mbps or more class began to be discussed for the first time in the U.S. Santa Clara Conference, September 1990, when the MPEG1 came to an end, resulting in the establishment of MPEG2 International Standard (IS) in the Singapore Conference, 1994.
The MPEG2 is composed of three small groups, or a system, video and audio. The formal names of these three international standards are ISO/IEC 13818-1 (system), ISO/IEC 13818-2 (video) and ISO/IEC 13818-3 (audio). At the present, the MPEG2 is the international common standard in the image compression field.
However, in a conventional MPEG2 encoder system, a signal of a raster scan format is applied to a pixel combiner in a mode determination unit, resulting in a large amount of time being required in combining pixel values. Further, the system becomes complex in construction, resulting in an increase in the installation cost thereof.
On the other hand, a conventional apparatus for converting a scan line format of a digital video signal is shown in U.S. Pat. No. 5,337,089, issued to Philips Electronics North America Corporation, New York, N.Y. The conventional scan line format conversion apparatus is adapted to limit a signal bandwidth to that of a transmission channel over which the video signal is to be transmitted to a receiver and to match an HDTV format with that of a standard NTSC signal. The conventional scan line format conversion apparatus is of particular utility for conversion between the presently contemplated HDTV format of 1050 scan lines per frame of paired interlaced fields at a field frequency of 60 Hz, and a progressive frame format of 525 scan lines per frame at a frame frequency of 30 Hz. To this end, the conventional scan line format conversion apparatus comprises first multiplexing means for performing an alternate multiplexing operation, a pair of transmission paths coupled respectively to respective outputs of the first multiplexing means so that even-numbered fields are supplied to a first of the paths and odd-numbered fields are supplied to a second of the paths, second multiplexing means for alternately outputting even-numbered fields transmitted by the first transmission path and odd-numbered fields transmitted by the second transmission path, delay means in the first transmission path for delaying each even-numbered field therein for substantially one field period and successively supplying the delayed even-numbered fields to the output of the first transmission path, low pass digital filter means in the second transmission path for vertically low pass filtering each odd-numbered field and successively supplying the filtered odd-numbered fields to the output of the second transmission path, and field store means coupled to the output of the second multiplexing means for receiving the delayed even-numbered fields and the filtered odd-numbered fields. The field store means is further adapted to produce subsampled fields at its output in synchronism with the even-numbered fields of the received signal, so that the subsampled fields constitute a digital output signal.
With the above-mentioned construction, the conventional scan line format conversion apparatus limits the signal bandwidth to that of the transmission channel over which the video signal is to be transmitted to the receiver and matches the HDTV format with that of the standard NTSC signal. However, the above-mentioned conventional scan line format conversion apparatus has a disadvantage in that it cannot reduce a pixel value of external input data.